If a variable is undef it will evaluate to an empty string when used in a regex. An empty string on the right side will match any string on the left.

For example: 

print "Match 1\n" if "teststring" =~ "";
print "Match 2\n" if "teststring" =~ undef;

__OUTPUT__
Match 1
Match 2

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In reply to Re: Help with null string behavior in regex? by Belgarion
in thread Help with null string behavior in regex? by McMahon

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