my @good_stuff = $stuff =~ /^START\n(.*?)^END$/gms;

# or alternatively with START and END around
my @good_stuff = $stuff =~ /(^START$.*?^END$)/gms;

##</code><code>##

$stuff = << EOF;
blah
START
a few lines
END
lots of lots of lines
EOF

##</code><code>##

$stuff = << EOF;
blah
START
lots of lots of lines
END
blah
EOF