use 5.010;
use strict;
use warnings;

sub sayit {
    say $1 // 'undef';
}

do {
    '42' =~ /(\d+)/ and sayit();
};
sayit();
__END__
42
undef

##</code><code>##

sub R {
   printf qq{before: \$_ is '$_'};
   printf qq{  \$1 is %s \n}, defined($1) ? qq{'$1'} : 'undefined';
   s/(\d+)// ? $1 + R() : 0;
   printf qq{after: \$_ is '$_'};
   printf qq{ \$1 is %s \n}, defined($1) ? qq{'$1'} : 'undefined';
}

$_ = 'x55x666x7777x1x';
R();
__END__
before: $_ is 'x55x666x7777x1x'  $1 is undefined 
before: $_ is 'xx666x7777x1x'  $1 is '55' 
before: $_ is 'xxx7777x1x'  $1 is '666' 
before: $_ is 'xxxx1x'  $1 is '7777' 
before: $_ is 'xxxxx'  $1 is '1' 
after: $_ is 'xxxxx' $1 is '1' 
after: $_ is 'xxxxx' $1 is '1' 
after: $_ is 'xxxxx' $1 is '1' 
after: $_ is 'xxxxx' $1 is '1' 
after: $_ is 'xxxxx' $1 is '1'