The regex needs to be anchored to the start of a line and followed by a word break. The conditional part also needs to account for lines that do not begin with 'If' at all.

#! /usr/bin/perl -w
use strict;
use warnings;

while (<DATA>)
{
    if (/^If(f?)\b/) {
        if ($1) {
            print "found Iff\n";
        }
        else {
            print "found If\n";
        }
    }
    else {
        print "no If\n";
    }
}


__DATA__
If and only iff
Iff this were the end
the end
last If
Iffy code or what
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