use strict;
use warnings;

my @lines = map {s/@/\t/g; "$_\n"} split(/\n/, << 'EOF');
@foo bar baz quux
 @foo bar baz quux
foo@bar baz quux
foo @bar baz quux
foo bar@baz quux
foo bar @baz quux
EOF

my $tabwidth = 8;

for my $line (@lines) {
    my $pos = 0;
    for my $part (split/(\t)/,$line) {
        if ($part eq "\t") {
            my $spaces = " " x ($tabwidth - ($pos % $tabwidth));
            $pos += length($spaces);
            print $spaces;
        } else {
            $pos += length $part;
            print $part;
        }
    }
}
__END__
        foo bar baz quux
        foo bar baz quux
foo     bar baz quux
foo     bar baz quux
foo bar baz quux
foo bar         baz quux