$_ is not a "regexp var". As for $`, $&, $', $1, etc, the variables are global, they will never go out of scope. The values however get either a new value (after a new succesful match), or revert to a previous value (on scope exit), whichever comes first:

"foo" =~ /([aeiou])/; say $1;
{
    "xyz" =~ /([aeiou])/; say $1; 
    "bar" =~ /([aeiou])/; say $1;
}
say $1;
__END__
o
o
a
o
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