command to layout tab separated list nicely

So, the answer becomes:

column -t file_name

Note that this splits columns at any whitespace, not just tabs. If you want to split on tabs only, use:

column -t -s $'\t' -n file_name

The -s $'\t' sets the delimiter to tabs only and -n preserves empty columns (adjacent tabs).

P.S.: Just want to point out that the credit goes to Alex as well. The original hint was provided by him as a comment to the question, but was never posted as an answer.

Here's a script to do it:

aligntabs.pl

#!/usr/bin/perl

my $delim = '\s*\t\s*';

my %length = ();
my @lines = ();
for my $line (<>) {
    chomp $line;
    my @words = split $delim, $line;
    my $numwords = scalar(@words);
    for my $i (0..$numwords-1) {
        my $maxlen = $length{$i} // 0;
        my $thislen = length($words[$i]);
        $maxlen = ($thislen > $maxlen)? $thislen: $maxlen;
        $length{$i} = $maxlen;
    }
    push @lines, [@words];
}

foreach my $wordsref (@lines) {
    my @words = @$wordsref;
    my $numwords = scalar(@words);
    for my $i (0..$numwords-1) {
        if ($i < $numwords-1) {
            my $fieldlen = $length{$i};
            printf "%-${fieldlen}s ", $words[$i];
        }
        else {
            print $words[$i];
        }
    }
    print "\n";
}

usage

$ aligntabs.pl < infile
var1                                     var2 var3
var_with_long_name_which_ruins_alignment var2 var3

For manual tab stops: expand -t 42,48

For automatic tab stops, as suggested by alex: column -t

(expand is on all POSIX systems. column is a BSD utility, available in many Linux distributions as well.)