use strict;
use warnings;

my $message =<<END;
<abc name="rn" value="42">
<form method="post" action="login.lp" name="authform" id="authform">
        <input type="hidden" name="rn" value="-1383135969">
<xyz name="rn" value="43">
END

$message =~ /action\s*=\s*"login.*?".*?"rn"\s+value\s*=\s*"(.+?)"/s;
my $rn = $1;
print "rn = '$rn'\n" if $rn;