Here is an portion of the xml file as requested.

<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>
[download]

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
[download]

I find XML::Simple to be very simple if the XML is very simple, but with non-trivial XML I am often surprised by what it produces and, if I persist with it, end up spending a lot of time re-reading the documentation and studying Data::Dumper dumps.

Here is an example which shows, step by step as I worked it out, how to access the source_name element...

use strict;
use warnings;
use Data::Dumper;

my $VAR1 = {
    'sas_residue_annotation' => [
        {
            'xmlns'   => 'http://www.ebi.ac.uk/WSsas/Schema',
            'sources' => [
                {
                    'source' => [
                        {
                            'source_name'  => [ '1iho' ],
                            'ref_evalue'   => [ '5.5e-50' ],
                            'ref_overlap'  => [ '282' ],
                            'ref_identity' => [ '47.20' ],
                            'ref_pmid'     => [ '11377204' ]
                        },
                        {
                            'source_name'  => [ '1mop' ],
                            'ref_evalue'   => [ '8.3e-38' ],
                            'ref_overlap'  => [ '264' ],
                            'ref_identity' => [ '43.60' ],
                            'ref_pmid'     => [ '12717031' ]
                        },
                        {
                            'source_name'  => [ '1n2b' ],
                            'ref_evalue'   => [ '8.2e-38' ],
                            'ref_overlap'  => [ '264' ],
                            'ref_identity' => [ '43.60' ],
                            'ref_pmid'     => [ '12717031' ]
                        },
                    ],
                },
            ],
        },
    ],
};

print Dumper($VAR1);
print Dumper($VAR1->{sas_residue_annotation});
print Dumper($VAR1->{sas_residue_annotation}[0]);
print Dumper($VAR1->{sas_residue_annotation}[0]{sources});
print Dumper($VAR1->{sas_residue_annotation}[0]{sources}[0]);
print Dumper($VAR1->{sas_residue_annotation}[0]{sources}[0]{source});

foreach my $source (@{$VAR1->{sas_residue_annotation}[0]{sources}[0]{s
+ource}}) {
    print "source name: ", join(',',@{$source->{source_name}}), "\n";
}
[download]

Note that I wrote each successive print after studying the output of the previous one. It is a bit tedious, but for deeply nested structures I find it faster than trying to do it all in my head and then debugging my errors.