!/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

##</code><code>##

Use of uninitialized value in concatenation (.) or string at ./828268.pl line 14, <DATA> line 1.
found 
Use of uninitialized value in pattern match (m//) at ./828268.pl line 16, <DATA> line 1.
found BAR

##</code><code>##

sub foo {
    # ...
    local $_ = "whatever";
    $_ = undef;
}

##</code><code>##

found FOO
found BAR