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
Prints...
sometext
some more text
some more in next line
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