In this situation i'd investigate whether regexes are even required. I bet the following is faster:

my %skip=map {$_=>1} qw(CPXX1 KLXX1 KMXX1 MEXX1 PAXX1 PMXX1 SLXX1 SZXX
+1 WXXX1 YZXX1);
while ( <IFILE>)
{
  print OFILE  unless $skip{substr($_,0,5)};
}
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