s/// does not untaint.

#!/usr/local/bin/perl -Tw

sub is_tainted {
    no warnings;
    no strict;
    return ! eval { join('',@_), kill 0; 1; };
}
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# is_tainted() was taken from perlfaq7

use strict;

my $site = <>;
chomp $site;
print is_tainted($site) ? "Tainted\n" : "Not tainted\n";

$site =~ s/[^-\@\w\/]//;
print is_tainted($site) ? "Tainted\n" : "Not tainted\n";

my $dir = "$sites_root/$site";
print is_tainted($dir)  ? "Tainted\n" : "Not tainted\n";

chdir($dir)
__END__
Tainted
Tainted
Tainted
Insecure dependency in chdir while running with -T switch at ./foo.pl 
+line 21, <> line 1.
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So, untaint $sites with an m// and $1, according to perlsec:

my $validchars = 'A-Za-z0-9_.-';
if ( $site =~ /^([$validchars]+)$/o ) {
    $site = $1;
}else{
    warn "Invalid site: $site\n";
    next;
}
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2;0 juerd@ouranos:~$ perl -e'undef christmas'
Segmentation fault
2;139 juerd@ouranos:~$
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