use strict;
use warnings;

my %strings;

foreach my $line (<DATA>) {
    $line =~ m/(\d+)/;
    my $class = $1;
    # Save the line in an array for this class of line
    # Keep references to the arrays in a hash keyed by
    # the class. Use push() to add each line to the array.
    push(@{$strings{$class}}, $line);
}


# Now iterate over the classes and print all the lines
# for each class.

foreach my $class (sort { $a <=> $b } keys %strings) {
    print "strings for class $class:\n";
    foreach my $string (@{$strings{$class}}) {
        print "\t$string";
    }
}
__DATA__
BLABLABLA 2 BLABLA
BLABLABLA 2 BLABLA
BLABLABLA 2 BLABLA
BLABLALBA 3 BLABLA
BLABLABLA 3 BLABLA