If you really want to sum them, what about

my $rh_count = { } ;
while ( <DATA> )
{
    my ( $string, $count ) = split;
    $rh_count->{$string} += $count;
}

print "$_: $rh_count->{$_}\n"
    for keys %$rh_count;

__DATA__
string1 10
string2 30
string1 0
string4 39
string9 345
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