as you already get sane and wise answers, I propose you a oneliner solution (change " to ' around the code to run it on Linux):

perl -lane "push @{$H{$F[0]}},$F[1]}{print map{$_,qq( ---> ),scalar @{
+$H{$_}},$/,(join $/,@{$H{$_}}),$/,$/}keys %H" data1.txt

0011 ---> 2
Sally
Roy

1122 ---> 2
Brandon
Simson

2233 ---> 1
George
[download]

You can use -MO=Deparse to have some clue on how to read it:

perl -MO=Deparse -lane "push @{$H{$F[0]}},$F[1]}{print map{ $_,qq( ---
+> ),scalar @{$H{$_}},$/,(join $/,@{$H{$_}}),$/,$/  }keys %H" 

BEGIN { $/ = "\n"; $\ = "\n"; }
LINE: while (defined($_ = readline ARGV)) {
    chomp $_;
    our @F = split(' ', $_, 0);
    push @{$H{$F[0]};}, $F[1];
}
{
    print map({$_, ' ---> ', scalar @{$H{$_};}, $/, join($/, @{$H{$_};
+}), $/, $/;} keys %H);
}
-e syntax OK
[download]

See perlrun for perl command line switches, but basically -l takes care of line ending (you are not chomp -ing lines!), -a does autosplit populating the @F array and -n wraps the code in a while loop without printing ( and -p does the same but printing). Braces in the part ..$F[1]}{print.. are a trick named Eskimo Greeting.

See perlvar for $/ and @F

You can explore these switches deparsing them one at time like in ( -e is for execute perl code, and -e 1 is just a null program):

perl -MO=Deparse -l -e 1 
BEGIN { $/ = "\n"; $\ = "\n"; }
'???';
-e syntax OK

perl -MO=Deparse -a -e 1

LINE: while (defined($_ = readline ARGV)) {
    our @F = split(' ', $_, 0);
    '???';
}
-e syntax OK

perl -MO=Deparse -n -e 1

LINE: while (defined($_ = readline ARGV)) {
    '???';
}
-e syntax OK

perl -MO=Deparse -p -e 1

LINE: while (defined($_ = readline ARGV)) {
    '???';
}
continue {
    die "-p destination: $!\n" unless print $_;
}
-e syntax OK
[download]