use strict;
use warnings;

use Data::Dumper;

my @paths = qw(
    This/is/the/full/path/file.abc.part-1.txt
    This/is/the/full/path/file.abc.part-2.txt
    This/is/the/full/path/file.abc.part-3.txt
    This/is/the/full/path/file.def.part-1.txt
    This/is/the/full/path/file.def.part-2.txt
    This/is/the/full/path/file.ghi.part-1.txt
    This/is/the/full/path/file.jkl.part-2.txt
    This/is/the/full/path/file.mno.part-5.txt
);

my %combo;

foreach my $pathfile ( @paths )
{
    if( $pathfile =~ m/^(.+file\.)(\w+?)\.(part-\d+)\.txt$/ )
    {
        push( @{ $combo{$1}{$2} }, $3 );
    }
    else
    {
        warn "$pathfile does not match expected format";
    }
}

foreach my $path ( keys %combo )
{
    foreach my $type ( keys %{ $combo{$path} } )
    {
        if( scalar @{ $combo{$path}{$type} } > 1 )
        {
            my $newfile = join( '', $path, $type, '.MERGED.txt' );
            print "These files go into $newfile:\n";
            print '   ', join( ', ', @{ $combo{$path}{$type} } ), "\n";
        }
    }
}

print Dumper( \%combo );

##</code><code>##

These files go into This/is/the/full/path/file.abc.MERGED.txt:
   part-1, part-2, part-3
These files go into This/is/the/full/path/file.def.MERGED.txt:
   part-1, part-2
$VAR1 = {
          'This/is/the/full/path/file.' => {
                                             'jkl' => [
                                                        'part-2'
                                                      ],
                                             'abc' => [
                                                        'part-1',
                                                        'part-2',
                                                        'part-3'
                                                      ],
                                             'mno' => [
                                                        'part-5'
                                                      ],
                                             'def' => [
                                                        'part-1',
                                                        'part-2'
                                                      ],
                                             'ghi' => [
                                                        'part-1'
                                                      ]
                                           }
        };