You have several problems.

• You are converting your 'NULL' strings to undefs before you do the substitutions to remove the quotes.

  If you had warnings enabled you would see that this causes two "Use of uninitialized value in substitution (s///) at ..." messages. One for each of the undefs.

  A simple solution to this is to reverse the order of your maps.

• Your substitution map is returning the result of the substitution hence the two 1s. One for each of the two values that are matched '196' and 'berg'.

  The simple answer here is to make sure that the result of the map (the last value in the code block) is $_.

• According to your text, you only want to strip 's around the number, but your substitution is also stripping them from 'berg'.

  This may just be words, but if you really only want to strip them from the number(s) then you should change thematch in the substitution to s/^\'(\d+?)\'$/....

Your also backwacking more than is necessary. Putting that all together we get

my $params = "'196', 'berg',NULL,NULL";
my @list_of_params = map { 
    $_ eq 'NULL' ? undef : $_ 
} map { 
    s/^'(\d+?)'$/$1/m; $_ 
} split /\s*,\s*/, $params ;
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It's probably not the way I would have done it, but it does correct some errors and produce the output you want.