To generate XML, your source data also needs to be structured.

Your data currently appears to be close to YAML format, so if you did convert it to YAML, you could do something like this:

use strict;
use warnings;
use YAML;

my $stuff = <<YAML;
---
ABC:
 DEF:
  - GHI
YAML

my $perl_structure = Load $stuff;

printit ($perl_structure);

sub printit{
  my $this = shift;
  if (ref $this eq "" ){
     print $this;
     return;
  }
  if (ref $this eq "ARRAY"){
     print "<value>";
       printit ($_) for @$this;
     print "</value>\n";
     return;
  }
  #print "Got " . ref ($this) . "\n";
  for my $k (keys %$this){
     print "<$k>\n";
     printit($_) for $this->{$k};
     print "</$k>\n";
  }
}
[download]

Output:

<ABC>
<DEF>
<value>GHI</value>
</DEF>
</ABC>
[download]

However - if your input is even slightly more complicated than this, I would recommend using a "real" XML generator module. (XML::Simple if you dont have experience with these).