BrowserUk,
While $a and $b shouldn't have anything to do with @_, let me explain anyway:

• The 1st time the function is called with no arguments, $i become 0 to compare the first value of the array.
• If the values of both arrays are equal at index $i, $_[0] is incremented by 1 which auto-vivifies into existence if necessary
• When goto &sub is invoked subsequently, @_ has a value which is used for $i

After explaining, I verified that $_[0] is behaving correctly in the b0rk version using the print statements. It still doesn't explain why $a and $b are undefined on the second invocation or why it blows up?

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