May be this is helpful:

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( $pathfile =~ m/^.+file\.\w+?\.part-\d+\.txt$/ ){
        (my $key = $pathfile) =~ s/part-\d+\.txt$/MERGED.txt/;
        push( @{ $combo{$key} }, $pathfile );
    }
    else{
        warn "$pathfile does not match expected format";
    }
}

print Dumper(\%combo);
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