$seed = $tab[0];

##</code><code>##

use warnings;
use strict;

open TAB, '<', 'test.txt' or die "Can not open file: $!";
my $seed = 1.04;
while (<TAB>) {
    chomp;
	my @tab = split / /;
	my $diff = $tab[0] - $seed;
	if ($diff >= 0.1) {
        print "tab[0]=$tab[0]\n";
        $seed = $tab[0];
    } 
}
close TAB;

##</code><code>##

3
5 6 7
2 22 222
10 9

##</code><code>##

tab[0]=3
tab[0]=5
tab[0]=10