Another approach to a regex solution, assuming Choroba's explanation of when a regex might be the appropriate tool... and multiple sources, simulated by the array below:

#!/usr/bin/perl
use 5.018;
use strict;
use warnings;

my @message = ('<form method="post" action="login.lp" name="authform" 
+id="authform">
                <input type="hidden" name="rn" value="-1383135969">',
              '<form method="post" action="login.lp" name="authform" i
+d="authform">
                <input type="hidden" name="rn" value="-2383135969">',
              '<form method="post" action="login.lp" name="authform" i
+d="authform">
                <input type="hidden" name="rn" value="-3383135969">');

for my $msg (@message) {
    $msg =~ /^action.*?\"rn\"\s+value\=+"([^"]+)"/g;
    # say " DEBUG: At Ln 15, msg: $msg";

    if ( $msg =~ /\"rn\"\s+value\=+"([^"]+)"/ ) {
    my $rn= $1; 
    say "At Ln 19, \$rn: $rn\n";
    }
}

=head

C:\>1119824.pl
At Ln 19, $rn: -1383135969

At Ln 19, $rn: -2383135969

At Ln 19, $rn: -3383135969

=cut
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