Check up on the return value of s///. In this case, your s/// always returns 1 (the number of substititions made in that iteration). map takes all those 1s and assigns them to @users. You need to return the changed value to map.

Also, your usage of map clobbers the value passed to map. True, it doesn't matter in this instance, but it's a bad habit.

my @users = map { my $s = $_;              # Protect the caller.
                  chomp($s);
                  $s =~ s/^\Q$MAIL_ROOT//;
                  $s } `find ...`;         # Return the changed value.
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Both List::MoreUtils and Algorithm::Loops provide a function that does this magic for you.

use List::MoreUtils qw( apply );
my @users = apply { chomp; s/^\Q$MAIL_ROOT//; } `find ...`;
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use Algorithm::Loops qw( Filter );
my @users = Filter { chomp; s/^\Q$MAIL_ROOT//; } `find ...`;
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Update: I fixed your regexp.