Why is this relatively simple regular expression so slow: 
/^ \s* $KEY /mx

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($KEY is always alphanumeric)
It is ten times slower than the almost-equivalent: 
/\n \s* $KEY /mx

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And also ten times slower than the I think equivalent (Could someone verify whether this is actually equivalent?) but much harder to read: 
/(?<![^\n]) \s* $KEY /mx

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The original regex is simple enough that a naive processing of it should give excellent performance. So what extra work is perl doing and why?

In reply to Inexplicably slow regex by Anonymous Monk

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