When I run your code, it seems to work for me. Why do you think it doesn't work? What output do you get if you print @list

use strict;
use warnings;
use Data::Dumper;

my $dsfe_mask = q{String: "00.55.55.00"};
$dsfe_mask =~ s/\n|STRING: //g;
$dsfe_mask =~ s/"+//g;
$dsfe_mask =~ s/\D+//g; 
my @list = split(//,$dsfe_mask);
print Dumper(\@list);

__END__
$VAR1 = [
          '0',
          '0',
          '5',
          '5',
          '5',
          '5',
          '0',
          '0'
        ];
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