Nothing. Maybe he was thinking of the following:

# XXX WRONG DON'T USE THIS
$re = qr/$re/i if $opt{i};
$re = qr/$re/m if $opt{m};
$re = qr/$re/s if $opt{s};
$re = qr/$re/x if $opt{x};
[download]

On the other hand, the following does work:

$re = "(?i:$re)" if $opt{i};
$re = "(?m:$re)" if $opt{m};
$re = "(?s:$re)" if $opt{s};
$re = "(?x:$re)" if $opt{x};
[download]