my $g_tab_width = 4;

my $s1 = "xxxx\tyyyy\tzzzz";
$s1 =~ s{(.*?)\t}{$1.(' ' x ($g_tab_width - length($1) % $g_tab_width))}ge;

my $s2 = "xxxx\tyyyy\tzzzz";
$s2 =~  s/(.*?)\t/$1    /g;

print $s1, "\n", $s2, "\n";