You could use something like my $l = length($string =~ s/\t/    /gr), though that's rather inefficient. Another solution:

my $tablength = 4;

my $string = "\tSample\ttext.";

my $expanded_length = render_length($string, $tablength);

sub render_length {
    my ($string, $tabl) = @_;
    my $length = length($string);
    my $tabcount = $string =~ tr/\t//;
    $length += ($tabl - 1) * $tabcount;
    return $length;
}
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This isn't terribly elegant (wrote it on the train on the way to work), but it should work fine, except that the entire notion of tab length is a choice that you make, and convention is ambiguous.