use warnings;
use strict;
use Fatal qw/ open close /;
use File::Set;

sub cback 
{
        my ($context, $prefix, $fpath, $ftype, $stat) = @_;
        if ($ftype eq 'f') {
                open my $fh, '<', $fpath;
                my $first_line = <$fh>;
                print $first_line;
        }
}

my $fs = File::Set->new();
$fs->add('/some/dir');
$fs->list('',\&cback);
[download]

prints the first line of every plain file in /some/dir and all its subdirs.

A callback is a reference to a subroutine; the important parameter passed to it is the third — full path to file.