Here's how you would select the information you want. I'll leave it as an exercise to modify the code to replace the values.

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

my $wholefile = '';

while(<DATA>)
{
    $wholefile .= $_;
}

while($wholefile=~m/\<default\>([^\>]+)\<\/default\>/gs)
{
    print qq~$1\n~;
}

1;


__END__
this is line 1
this is line 2
<default>sometext</default>
this is line 3
<default>some more text
some more in next line
</default>
this is last line
[download]

Prints...

sometext
some more text
some more in next line
[download]