The regex won't match because there are quotes in there that don't appear in the input.

This seems to work: 

use strict;
use warnings;

while (<DATA>)
{
    print "$1\n" if m/GO:(\d+)/
}

__DATA__

dd class="xref">GeneID:947412</dd>
                        
                    
                    <dd class="xref">GenomeReviews:AP009048_GR</dd>
                        
                    
                    <dd class="xref">GenomeReviews:U00096_GR</dd>
                        
                    
                    <dd class="xref"><a href="http://amigo.geneontolog
+y.org/cgi-
+bin/amigo/term-details.cgi?term=GO:0005524">GO:0005524</a></dd>
                        
                    
                    <dd class="xref"><a href="http://amigo.geneontolog
+y.org/cgi-
+bin/amigo/term-details.cgi?term=GO:0005886">GO:0005886</a></dd>
                        
                    
                    <dd class="xref"><a href="http://amigo.geneontolog
+y.org/cgi-
+bin/amigo/term-details.cgi?term=GO:0006810">GO:0006810</a></dd>
                        
                    
                    <dd class="xref"><a href="http://amigo.geneontolog
+y.org/cgi-
+bin/amigo/term-details.cgi?term=GO:0016301">GO:0016301</a></dd>
                        
                    
                    <dd class="xref">HOGENOM:P27254</dd>

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In reply to Re: Help regarding regular expression by moritz
in thread Help regarding regular expression by chavanak

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