sub re_p2js {
  local @_ = @_;
  for( @_ ) {
    1 while s[^\(\?(\w*)-?\w*:(.*)\)$][/$2/$1]
  }
  @_
}

print for re_p2js qr/abcde/, qr/(1|2)z$/, qr/^abcd*/ims
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This will extract the outer (?-xims:) -- and it may be enough for your purposes but, as others already posted, it will not compile perl regexen into js regexen.

UPDATE: changed to respect the flags. The output will be:

/abcde/
/(1|2)z$/
/^abcd*/msi
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[]s, HTH, Massa (κς,πμ,πλ)

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