#!/usr/bin/perl
use strict;
use warnings;
use 5.016;

# Modified from parent and MidLifeXis' 1043331

open my $in, '<', "a.txt";
print firstline("b.txt");

sub firstline {
    say "at_: @_";
    my $file = shift;
    my $retval = open (my $FH, '<', $file );
    $retval && return scalar <$FH>;
  # no close() required
}