The string following whatever was matched by the last successful pattern match. (See "Scoping Rules of Regex Variables"). Example:
local $_ = 'abcdefghi'; /def/; print "$`:$&:$'\n"; # prints abc:def:ghi
See "Performance issues" above for the serious performance implications of using this variable (even once) in your code.
This variable is read-only, and its value is dynamically scoped.
' often follows a quoted string.