package List::Merger;
@ISA         = qw(Exporter);
@EXPORT      = qw();
@EXPORT_OK   = qw(gen_merger);

use strict;
use warnings;
use List::Util 'first';

our $VERSION = '0.01';

sub gen_merger {
    my ($list, $fetch, $compare, $finish) = @_;
    my @item = map $fetch->($_), @$list;
    
    my $done;
    return sub {
        return $finish if $done;        
        my $idx  = first {$item[$_] ne $finish} 0 .. $#item;
        my $next = $item[$idx];
        for ($idx + 1 .. $#item) {
            next if $item[$_] eq $finish;
            my $result = $compare->($next, $item[$_]);
            $next = $item[$_] if $result == 1;
        }
        $item[$idx] = $fetch->($list->[$idx]);
        $done = 1 if ! first {$item[$_] ne $finish} $idx .. $#item;
        return $next;
    };
}
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my $finish   = 'A val that is guaranteed not to be present in any list
+';
my @list     = (\@arr1, \@arr2, \@arr3, \@arr4, \@arr5);
my $fetch    = sub {
    my $item = shift @_;
    return $finish if ! $@item;
    return shift $@item;
};
my $compare  = sub {
    my ($item1, $item2) = @_;
    return uc($item1) cmp uc($item2);
}
my $next = gen_merger(\@list, $fetch, $compare, $finish);
while (1) {
    my $item = $next->();
    last if defined $item && $item eq $finish;
    print "$item\n";
}
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my $finish   = 'A val that is guaranteed not to be present in any list
+';
my @list     = ($fh1, $fh2, $fh3, $fh4, $fh5);
my $fetch    = sub {
    my $fh = shift @_;
    return $finish if eof $fh;
    return scalar <$fh>;
};
my $compare  = sub {
    my ($line1, $line2) = @_;
    my ($stamp1) = $line1 =~ /^(\d+)/;
    my ($stamp2) = $line2 =~ /^(\d+)/;
    return $stamp1 <=> $stamp2;
}
my $next = gen_merger(\@list, $fetch, $compare, $finish);
while (1) {
    my $item = $next->();
    last if defined $item && $item eq $finish;
    print "$item\n";
}
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