Just read each line, remove the line terminator (chomp) then split name and value and add value to the appropriate hash element.

$ perl -Mstrict -Mwarnings -MData::Dumper -E '
open my $dataFH, q{<}, \ <<EOF or die $!;
john, 100
barry, 300
john, 200
mary, 150
barry, 200
EOF

my %names;
while ( <$dataFH> )
{
    chomp;
    my( $name, $value ) = split m{\s*,\s*};
    $names{ $name } += $value;
}

print Data::Dumper->Dumpxs( [ \ %names ], [ qw{ *names } ] );'
%names = (
           'john' => 300,
           'barry' => 500,
           'mary' => 150
         );
$
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I hope this is helpful but ask further if you need more explanation.