Thank you Corion ! The code now works :

#!/usr/bin/perl -w
use strict;
use warnings;


my $path = join ';', (
    'D:\aa\bb\cc',
    'D:\dd\ee\ff',
    'D:\gg\hh\ii',
);

my $path_component = quotemeta( 'D:\aa\bb\cc;' );

print $path, "\n\n";

$path =~ s/$path_component//i;

print $path, "\n\n";
[download]

... and, more importantly, I've now understood what was going wrong...