Lacking a few details, but here is how I would approach this:

use strict;
use XML::Simple;

my $xs = XML::Simple->new(KeepRoot=>1);
my $data;
{
  local $/;
  $data = <DATA>;
}

my $inStruct = $xs->XMLin($data);
my $outStruct;

foreach my $tag (@{$inStruct->{Root}->{'dc:subject'}})
{
  my @matches = $tag->{content} =~ /(\d+th century)/g;
  foreach my $match (@matches)
  {
    my %item = %$tag;
    $item{content} = $match;
    push(@{$outStruct->{Root}->{'dc:subject'}}, \%item);
  }
  
}

my $resultXML = $xs->XMLout($outStruct);
print $resultXML;

__DATA__
<Root>
<dc:subject xml:lang="en-US">18th century 17th century</dc:subject>
<dc:subject xml:lang="en-US">18th century 14th century</dc:subject>
<dc:subject xml:lang="en-US">18th century 12th century</dc:subject>
<dc:subject xml:lang="en-US">18th century 11th century</dc:subject>
<dc:subject xml:lang="en-US">18th century 10th century</dc:subject>
<dc:subject xml:lang="en-US">18th century 12th century</dc:subject>
<dc:subject xml:lang="en-US">18th century 12th century 12th century 12
+th century 12th century</dc:subject>
</Root>
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