Your next step is to inspect what is in %CONFIG_FILE when you encounter that condition.

Use the following code:

       if ($file =~ /$key/) {
           $mapattach = $value;
           use Data::Dumper;
           print Dumper \%CONFIG_FILE;
        }
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Also output whenever you get a match on /$key/:

       if ($file =~ /$key/) {
           $mapattach = $value;
           use Data::Dumper;
           print Dumper \%CONFIG_FILE;
           print "'$file' matches /$key/, setting value '$value'\n";
        }
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Maybe there are two keys in %CONFIG_FILE that match the filename and the second one overwrites the first match in some cases? You are aware that hash keys are not ordered in Perl?

OK, took some finding but this might be the problem. The blank lines in your config file are creating a blank key entry in the %CONFIG_PARAMS hash i.e. "" => undef. When you loop through the keys at some point the match becomes

 if ($file =~ //) {
   $mapattach = undef;
 }
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Because the order is random it depend which keys follow as to whether it gets corrected. The fix is to add a line to skip the blanks lines in here

# parse the config file
while (<FILE>) {

  # remove leading spaces
  $_ =~ s/^\s+//g;

  # remove trailing spaces
  $_ =~ s/\s+$//g;

  # ignore comments
  next if $_ =~ /^#/;  

  # ignore blanks
  next unless /\S/;

  my @params = split /=/ , $_;

  $CONFIG_PARAMS{$params[0]}=$params[1];

}
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A better way would be use a regex to extract the key from the filename and match directly like this.

my $mapattach;
# add () inside // to capture match
if ($file !~ /($CONFIG_PARAMS->{'OUTCRIT'})/) {
  Println "Skipping $file, doesn't match $CONFIG_PARAMS->{'OUTCRIT'}";
  next OUTBOUND;
} else {
  $mapattach = $CONFIG_PARAMS->{$1};
}
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Thank you Poj, Corion and all the Monks who took there valuable time to inspect code and gave the solution.

Ignore blanks has worked for me.

  
# ignore blanks
  next unless /\S/;
[download]

Thanks & Regards

Sri