Even string eval doesn't help, as plain interpolation of the $replacement can break if it contains a slash

My first thought was to use eval but I hit a brick wall when I tried...
I was thinking of first changing a slash in $replacement for a double slash then using eval to do the substitution but I got stuck getting the result of the substitution.

$replacement =~ s/\\/\\\\/g;
$string =~ eval "s/\$pattern/\$replacement/$flags";
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But that doesn't do it...