my $source = << 'END';
adf
<!-- InstanceBeginEditable name="guts" -->
this is the part we want
<!-- InstanceEndEditable -->
adf
<!-- InstanceBeginEditable name="crap" -->
adf
<!-- InstanceEndEditable -->
adf
END

# Pull out just what we need
my ($new_source) = $source =~ /<!-- InstanceBeginEditable name="guts" -->(.?)<!-- InstanceEndEditable -->/s;