# some sample data:
my @rep1_pids = (1..10);
my @rep2_pids = (11..20);
my @rep3_pids = (21..30);
my @pairs = ( "11\t5", "22\t14", "4\t8", "1\t29" );

# organize sources into HoH (%arrayhash):
# create %hashbuilder first, to assign source names to arrays
 
my %hashbuilder = ( rep1 => \@rep1_pids,
                    rep2 => \@rep2_pids,
                    rep3 => \@rep3_pids,
                   );

# for each source, array names will be the primary hash keys,
# and the array values will become keys of the secondary hashes:

my %arrayhash;
for my $src ( keys %hashbuilder ) {
    $arrayhash{$src}{$_} = 1 for ( @{$hashbuilder{$src}} );
}

# now identify sources for the members (items) in each pair:

for my $pairstr ( @pairs ) {
    my $srcs = '';
    for my $item ( split( /\t/, $pairstr )) {
        for my $src ( sort keys %arrayhash ) {
            if ( $arrayhash{$src}{$item} ) {
                $srcs .= "$item is from $src; ";
            }
        }
    }
    print $srcs, "\n";
}