How to write procedure which will loop the input line N times in TCL

The natural way of doing that in Tcl would be this:

proc n_range {varName from to body} {
    upvar 1 $varName var
    for {set var $from} {$var <= $to} {incr var} {
        uplevel 1 $body
    }
}

n_range i 0 5 {
    puts $fp "abc$i"
}

The upvar and uplevel commands are a major part of the secret sauce of Tcl; they let you create your own control constructs in very little code.

Here's the "substitute and collect" approach. Less general than Donal's solution, but closer to what's requested, and not significantly more complicated IMHO:

proc string_for {string count} {
    for {set i 1} {$i <= count} {incr i} {
        append result [subst $string]
    }
    return $result
}

puts $fp [string_for {abc$i\n} 5]