That code stores in $1 a copy of $b with everything after the last "_" removed, including the "_" itself.

There are exceptions.
$1 is left unchanged if $b contains no "_".
$1 is left unchanged if $b contains nothing before the last "_".
$1 is left unchanged if $b contains nothing after the last "_".

When executed in list context, it returns $1 on match or an empty list on failure to match.

In reply to Re: What this pattern matches by ikegami
in thread What this pattern matches by Anonymous Monk

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