Binary "=~" binds a scalar expression to a pattern match. Certain
operations search or modify the string $_ by default. This operator
makes that kind of operation work on some other string. The right
argument is a search pattern, substitution, or transliteration. The left
argument is what is supposed to be searched, substituted, or
transliterated instead of the default $_. When used in scalar context,
the return value generally indicates the success of the operation.
Behavior in list context depends on the particular operator. See "Regexp
Quote-Like Operators" for details and perlretut for examples using these
operators.

If the right argument is an expression rather than a search pattern,
substitution, or transliteration, it is interpreted as a search pattern
at run time.