!/usr/bin/perl -l
use strict;
use warnings;

sub foo {
    # ...
    $_ = undef;
}

while (<DATA>) {
    chomp;
    if (/^FOO/) {
        foo();
        print "found $_";
    }
    if (/^BAR/) {
        # ...
        print "found $_";
    }
    # ...
}

__DATA__
FOO
BAR