The string following whatever was matched by the last successful pattern match (not counting any matches hidden within a BLOCK or eval() enclosed by the current BLOCK). (Mnemonic:
' often follows a quoted string.) Example:
$_ = 'abcdefghi'; /def/; print "$`:$&:$'\n"; # prints abc:def:ghi
This variable is read-only and dynamically scoped to the current BLOCK.
The use of this variable anywhere in a program imposes a considerable performance penalty on all regular expression matches. See BUGS.