while( $string =~ m/\bstart:(.+?)\bstop:/g ) {
    print "$1\n";
}

##</code><code>##

my $lastmatch = ( $string =~ m/\bstart:(.+?)\bstop:/g )[ -1 ];

##</code><code>##

if( $string =~ m/\bstart:(?!.+?(?:\bstart:))(.+?)\bstop:/ ) {
    print "$1\n";
}