use strict;
my $str = 'ABC//DEG//IJK//LMN//OPQ//rstuvwxyz
BCA//EGD//JKI//MNL//PQO//stuvwxyzr
ACB//DGE//IJK//LNM//OQP//rstuvwxyz
ABC//DEG//IJK//LMN//OPQ//rstuvwxyz
CAB//GDE//KIJ//NLM//QOP//rstuvwxyz
BAC//EDG//JIK//MLN//POQ//rstuvwxyz';
my $regex = qr`^(?:[A-Z]+//){2}[A-Z]+\K//`mp;
my $subst = '\\\\\\\\';
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