use strict;
use warnings;
my $input = <<HERE;
log1    123
log1    233
log1    223
log2    465
log2    231 
log2    456
HERE

my %groups;
{
    my @items = split /\s+/, $input;
    while ( @items ) {
        push @{$groups{ +shift( @items ) }}, shift( @items );
    }
}
{
    local $" = "\t";
    print "$_\n@{$groups{$_}}\n\n" foreach sort keys %groups;
}

__OUTPUT__

log1
123     233     223

log2
465     231     456