Since you are new to XML, allow me to offer a different approach, based on XML::Twig, which, in my opinion, is no more difficult to learn than XML::Simple, and will work for a wider range of XML structures:

use strict;
use warnings;
use XML::Twig;

my $xmlStr = <<XML;
<sas_residue_annotation xmlns="http://url/Schema" xmlns:xsi="http://ww
+w.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://url/Sche
+ma WSsas.xsd">>
<sources>
<source>
<source_name>1iho</source_name>
<ref_identity>47.20</ref_identity>
<ref_overlap>282</ref_overlap>
<ref_evalue>5.5e-50</ref_evalue>
<ref_pmid>11377204</ref_pmid>
</source>
<source>
<source_name>1mop</source_name>
<ref_identity>43.60</ref_identity>
<ref_overlap>264</ref_overlap>
<ref_evalue>8.3e-38</ref_evalue>
<ref_pmid>12717031</ref_pmid>
</source>
</sources>
</sas_residue_annotation>
XML

my $twig= new XML::Twig(
    twig_handlers => { source_name => \&source_name }
);
$twig->parse($xmlStr);
exit;

sub source_name {
    my ($twig, $name) = @_;
    print $name->text(), "\n";
}  

__END__
1iho
1mop
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