Collect your data in a hash of arrays, pushing on the data as it is seen,

my %hash;
while (<IN>) {
    chomp;
    my @foo = split /, /;
    push @{$hash{$foo[0]}}, 0+$foo[1];
}
for (sort keys %hash) {
    print OUT join ', ', $_, @{$hash{$_}};
    print OUT $/;
}
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The 0+$foo[1] is there to make the value numeric and so strip trailing whitespace.