No modules and quicker than a regex, faster than a speeding bullet, makes you popular with the girls....:)

#! perl -sw
use strict;

my $tabn = 8;
my $tabc = '.'; #! Use a space, '.' is just for demonstration.
while( <DATA> ) {
    print;
    my $p; substr($_,$p,1) = $tabc x ($tabn - $p % $tabn) while ($p=in
+dex($_, "\t"))>-1;
    print;
}

__DATA__
the quick brown fox
    the    quick    brown    fox
        the        brown        fox
[download]

Gives

c:\test>203941
the quick brown fox
the quick brown fox
        the     quick   brown   fox
........the.....quick...brown...fox
                the             brown           fox
................the.............brown...........fox

c:\test>
[download]

Note: The original data contained tabs....honest! I just couldn't think of an easy way to show them.

Note 2: If it doesn't look like the output is correct, you are probably viewing it with a proportional font.