sub f_bad   {                   'b' =~ /(.*)/;   return $_[0]; }
sub f_good1 {   my ($var) = @_; 'b' =~ /(.*)/;   return $var;  }
sub f_good2 { { local $1;       'b' =~ /(.*)/; } return $_[0]; }

'a' =~ /(.*)/; print f_bad($1),   "\n";  # b
'a' =~ /(.*)/; print f_good1($1), "\n";  # a
'a' =~ /(.*)/; print f_good2($1), "\n";  # a