my old code matches the ;doc and gives result such as ssscument.write('<iframe ...shortened...</iframe>');

No, it does not:

use 5.010;
use strict;
use warnings;

$_ = q[;document.write('<iframe src="any url" scrolling="auto" framebo
+rder="no" align="center" height="15" width="15"></iframe>');];
s/(\;do.*)(\<\/iframe\>\'\)\;)/sss/;
say
__END__
sss
[download]

Anyway, it's easier if you use a different delimiter so that you don't have to escape the forward slashes:

s{(\;do.*)(</iframe>'\);)}{//eof};
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