use strict;
use warnings;

my @R; # Constructing your matrix...
while (<DATA>) { chomp; push @R, [split(/ /)]; }

for (@R) {
    if ($_->[0] eq 'a') {
        unshift @$_, '#'; print join("\t", @$_)."\n";
    }
}

__DATA__
x y z
a b c
e f g
h i j