Well, is the input in the form of one big string, or is it in some other form? I'll assume it's a string.

my $input = "log1 123 log1 233 log1 223 log2 465 log2 231 log2 456";
my @items = split /\s+/, $input;
my %groups;
while ( @items ) {
    push @{$groups{ +shift( @items ) }}, shift( @items );
}
print "$_ @{$groups{$_}}\n" foreach sort keys %groups;
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