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use Text::CSV;

...
my $string = $csv->string();

print $string;

my $inputstring = qq{field1\tfield2\t\"field3\t\t\"\tfield4\n};

$inputstring =~ s{("(?: (?> [^\"\\]+ ) | \\ . )*")}{ (my $str = $1) =~
+ s/\t//g; $str }xesg;

print $inputstring;