Next time please show the code you tried: generally monks prefere (with reason) to see some effort from the wisdom seeker.

You already got wise answers and smart ones too. Mine is a oneliner (be aware of windows double quotes: use single quote on Linux).

perl -F"," -lanE "$h{join ',',@F[0,1]}+=$F[2]}{map{say $_.','.$h{$_}}s
+ort keys %h"  sample.csv

3211111,100,4.4
3211112,100,2.5
3211112,101,3.2
3211113,100,5.2
[download]

See perlrun to know how many useful switches and parameter you can feed to Perl!

Basically speaking -a autosplits incoming strings (at spaces), feeding the special @F array (see perlvar for it)

-F specify an alternative pattern for the autosplit

-l uses a smart line handling

-n wraps your program into a while loop without printing his input ( -p also print it)

-E executes the following code and import some feature (like say I used). Normally you can use -e

}{ is a trick: see eskimo greeting

If the oneliner seems overhelming for you, use -MO=Deparse to have it expanded:

perl -MO=Deparse -F"," -lanE "$h{join ',',@F[0,1]}+=$F[2]}{map{say $_.
+','.$h{$_}}sort keys %h"  sample.csv


BEGIN { $/ = "\n"; $\ = "\n"; }
BEGIN {
    $^H{'feature_unicode'} = q(1);
    $^H{'feature_say'} = q(1);
    $^H{'feature_state'} = q(1);
    $^H{'feature_switch'} = q(1);
}
LINE: while (defined($_ = <ARGV>)) {
    chomp $_;
    our(@F) = split(/,/, $_, 0);
    $h{join ',', @F[0, 1]} += $F[2];
}
{
    map {say $_ . ',' . $h{$_};} sort(keys %h);
}
-e syntax OK
[download]

If you are really lazy you can learn what switches do using MO=Deparse adding them progressively and seeing what happens executing a noprogram ( is what perl -e 1 is, marked by '???' in the deparsed output):

perl -MO=Deparse -e 1
'???';
-e syntax OK

perl -MO=Deparse -n -e 1
LINE: while (defined($_ = <ARGV>)) {
    '???';
}
-e syntax OK

perl -MO=Deparse -n -a -e 1
LINE: while (defined($_ = <ARGV>)) {
    our(@F) = split(' ', $_, 0);
    '???';
}
-e syntax OK

perl -MO=Deparse -n -a -F"," -e 1
LINE: while (defined($_ = <ARGV>)) {
    our(@F) = split(/,/, $_, 0);
    '???';
}
-e syntax OK

perl -MO=Deparse -n -a -F"," -l -e 1
BEGIN { $/ = "\n"; $\ = "\n"; }
LINE: while (defined($_ = <ARGV>)) {
    chomp $_;
    our(@F) = split(/,/, $_, 0);
    '???';
}
-e syntax OK

perl -MO=Deparse -n -a -F"," -l -E 1
BEGIN { $/ = "\n"; $\ = "\n"; }
BEGIN {
    $^H{'feature_unicode'} = q(1);
    $^H{'feature_say'} = q(1);
    $^H{'feature_state'} = q(1);
    $^H{'feature_switch'} = q(1);
}
LINE: while (defined($_ = <ARGV>)) {
    chomp $_;
    our(@F) = split(/,/, $_, 0);
    '???';
}
-e syntax OK
[download]

L*