my $header = <>;
chomp($header);
my @field_names = split  /\t/, $header;

my %grouped_by_num2;
while (<>) {
    chomp;
    my %rec; @rec{ @field_names } = split /\t/;
    push @{ $grouped_by_num2{$rec{SECOND_NUMBER}} }, \%rec;
}

use Data::Dumper;
print(Dumper(\%grouped_by_num2));