Can you "ignore" a file in Perforce?

As of version 2012.1, Perforce supports the P4IGNORE environment variable. I updated my answer to this question about ignoring directories with an explanation of how it works. Then I noticed this answer, which is now superfluous I guess.

Assuming you have a client named "CLIENT", a directory named "foo" (located at your project root), and you wish to ignore all .dll files in that directory tree, you can add the following lines to your workspace view to accomplish this:

-//depot/foo/*.dll //CLIENT/foo/*.dll
-//depot/foo/.../*.dll //CLIENT/foo/.../*.dll

The first line removes them from the directory "foo" and the second line removes them from all sub directories. Now, when you 'Reconcile Offline Work...', all the .dll files will be moved into "Excluded Files" folders at the bottom of the folder diff display. They will be out of your way, but can still view and manipulate them if you really need to.

You can also do it another way, which will reduce your "Excluded Files" folder to just one, but you won't be able to manipulate any of the files it contains because the path will be corrupt (but if you just want them out of your way, it doesn't matter).

-//depot/foo.../*.dll //CLIENT/foo.../*.dll

Will's suggestion of using .p4ignore only seems to work with the WebSphere Studio (P4WSAD) plugin. I just tried it on my local windows box and any files and directories that I listed were not ignored.

Raven's suggestion of modifying your client spec is the correct way under Perforce. Proper organization of your code/data/executables and generated output files will make the process of excluding files from being checked in much easier.

As a more draconian approach, you can always write a submit trigger which will reject submission of change-lists if they contain a certain file or files with a certain extension, etc.

I have found it easiest to reconcile offline work using a BASH script like this one:

#!/bin/bash
# reconcile P4 offline work, assuming P4CLIENT is set
if [ -z "$P4CLIENT" ] ; then echo "P4CLIENT is not set"; exit 1; fi
unset PWD # confuses P4 on Windows/CYGWIN

# delete filew that are no longer present
p4 diff -sd ... | p4 -x - delete

# checkout files that have been changed.  
# I don't run this step.  Instead I just checkout everything, 
# then revert unchanged files before committing.
p4 diff -se ... | pr -x - edit

# Add new files, ignoring subversion info, EMACS backups, log files
# Filter output to see only added files and real errors
find . -type f \
 | grep -v -E '(\.svn)|(/build.*/)|(/\.settings)|~|#|(\.log)' \
 | p4 -x - add \
 | grep -v -E '(currently opened for add)|(existing file)|(already opened for edit)'

I adapted this from this Perforce Knowledge Base article.

If you are using the Eclipse Perforce plugin, then the plugin documentation lists several ways to ignore files.