map returns the result of its expression and s/// returns either TRUE or FALSE so your array is receiving these true or false values. You need to return $_ after s/// has modified it:

chomp( my @users = map { s/$MAIL_ROOT//; $_ } `find $MAIL_ROOT -type d
+ -maxdepth 1` );
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However what you really should be doing is:

$MAIL_ROOT = '/var/mail';

opendir my $dh, $MAIL_ROOT or die "Cannot open '$MAIL_ROOT' $!";

my @users = grep -d && !/\A\.\.?\z/, readdir $dh;

closedir $dh;
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