if ($Bin =~ m!^(/home/someuser/somewebsite/[a-z]{2,4}/cgi-bin/)! ) {
   $Bin = $1;
} else {
   die "Go away, stupid script kiddie!\n";
}

# or maybe:

$Bin =~ m!^(/home/someuser/somewebsite/[a-z]{2,4}/cgi-bin/)! or die "Where am I?\n";
$Bin = $1;