Your problem statement is weak and it is hard for me to see how your code could work. But it could be that something like this is what you want.

#!/usr/bin/perl -w
use strict;

my %add2stanza = (  '12.34.56.79' => "NEW DATA for 12.34.56.79",
                    "12.34.56.80" => 'More new data for 12.34.56.80',
                 );
                
while (<DATA>)
{
    print;
    if (m/\<stanza\s+(.*)\>/)
    {
       my $number = $1;
       add_data($add2stanza{$number}) if $add2stanza{$number};
    }
}

# not the best loop structure as the return is buried within 
# the loop but as a quick hack, shows some points about how
# to do this...
sub add_data
{
   my $new_data = shift;
   while (<DATA>)
   {
       if (/\<\/stanza\>/)
       {
          print "    $new_data\n";
          print '</stanza>',"\n";
          return;
       }
       else
       {
          print;
       }
   }
}

=prints
<stanza 12.34.56.78>
    sl1  this is my line
    sl2  justanotherline
</stanza>
<line  1224>
    data
</line>    
<stanza 12.34.56.79>
    sl1  this is my line
    sl2  justanotherline
    NEW DATA for 12.34.56.79
</stanza>
<stanza 12.34.56.80>
    sl1  this is my line
    sl2  justanotherline
    More new data for 12.34.56.80
</stanza>
<stanza 12.34.60.90>
    sl1  this is my line
    sl2  justanotherline
</stanza>
=cut


__DATA__
<stanza 12.34.56.78>
    sl1  this is my line
    sl2  justanotherline
</stanza>
<line  1224>
    data
</line>    
<stanza 12.34.56.79>
    sl1  this is my line
    sl2  justanotherline
</stanza>
<stanza 12.34.56.80>
    sl1  this is my line
    sl2  justanotherline
</stanza>
<stanza 12.34.60.90>
    sl1  this is my line
    sl2  justanotherline
</stanza>
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