#! 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=index($_, "\t"))>-1;
	print;
}

__DATA__
the quick brown fox
	the	quick	brown	fox
		the		brown		fox

##</code><code>##

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>