my @users = map { my $s = $_;              # Protect the caller.
                  chomp($s);
                  $s =~ s/^\Q$MAIL_ROOT//;
                  $s } `find ...`;         # Return the changed value.

##</code><code>##

use List::MoreUtils qw( apply );
my @users = apply { chomp; s/^\Q$MAIL_ROOT//; } `find ...`; 

##</code><code>##

use Algorithm::Loops qw( Filter );
my @users = Filter { chomp; s/^\Q$MAIL_ROOT//; } `find ...`;