Given that you have fixed strings, eq and ne are likely better choices than regular expressions. Also, note that your code has assumptions about depth in it - the fact that you had to turn off warnings is usually a sign you have a design issue. Perhaps something more like:

use strict;
use warnings;
use Data::Dumper;


my @last = ();
while(<DATA>) {
    chomp;
    my @paths = split /\|/;
    
    my $different = 0;
    foreach my $i (0 .. $#last) {
        last if $i > $#paths;
        if ($different or $last[$i] ne $paths[$i]) {
            $different = 1;
            print join('|',@paths[0 .. $i]),"\n";
        }
    }
    
    foreach my $i ($#last+1 .. $#paths) {
        print join('|',@paths[0 .. $i]),"\n";
    }

    @last = @paths;
}

__DATA__
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