What's with "\s*?"? Don't you simply mean "\s*"?

Anyway,

s/
    (?<!_)
    ( split \s* \( \s* )
    (
        " (?:[^\\"]+|\\.)* "
    |
        ' (?:[^\\']+|\\.)* '
    )
/
    my @x = ($1,$2);
    $x[1] =~ s{/}{\\/}g;
    "preg_$x[0]$x[1]"
/xesg
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Untested.

Will create an error if the "/" is already escaped.