use strict;
my $str = 'it.mycompany.db.beans.str1.PD_T_PD_CLASS
it.mycompany.db.beans.join.PD_T_CLASS
it.mycompany.db.beans.str2.PD_T_CLASS_1
it.mycompany.db.beans.join.PD_T_CLASS_1
PD_T_CLASS myVar = new PD_T_CLASS();
myVar.setPD_T_CLASS(something);';
my $regex = qr/(?:\G(?!\A)|^(?!.*\.join\.))(.*?)PD_/mp;
my $subst = '$1';
my $result = $str =~ s/$regex/$subst/rg;
print "The result of the substitution is' $result\n";
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for Perl, please visit: http://perldoc.perl.org/perlre.html