The problem is outside what you showed us here, I bet. What you showed works fine (as seen below). Could you provide more of the code?

$outdata = <<'__EOI__';
<cfmail to="#to_address#">
    ......
</cfmail>
__EOI__

$outdata =~ s{<cfmail}{<!--- <cfmail}g;
$outdata =~ s{</cfmail>}{</cfmail> --->}g;

print($outdata);

__END__
output
======
<!--- <cfmail to="#to_address#">
    ......
</cfmail> --->
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