Further to davido's post:
bayareamonk: You say you have Perl 5.10 available (which provides \K), so here's a small simplification:

c:\@Work\Perl\monks>perl -wMstrict -le
"use Regexp::Common 'net';
 ;;
 my $input = qq{<yoda yoda> <ip-compute-10.10.10.1-internal> <yadda ya
+dda> \n}
           . qq{<more stuff>}
           ;
 print qq{[[$input]] \n};
 ;;
 my $want = qq{<yoda yoda> <ip-compute-10.10.25.18-internal> <yadda ya
+dda> \n}
          . qq{<more stuff>}
          ;
 ;;
 my $replacement_ip = '10.10.25.18';
 ;;
 $input =~ s{ <ip-compute- \K $RE{net}{IPv4} (?= -internal>) }
            {$replacement_ip}xmsg;
 ;;
 print $input eq $want ? 'Success:' : 'Failure:';
 print qq{[[$input]]};
"
[[<yoda yoda> <ip-compute-10.10.10.1-internal> <yadda yadda>
<more stuff>]]

Success:
[[<yoda yoda> <ip-compute-10.10.25.18-internal> <yadda yadda>
<more stuff>]]
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