Seems to behave correctly for me:

#!perl -l

use strict;
use warnings;

my %hash;
my $string = '2e-149';

$hash{foo} = 1e-149;
print "\$hash{foo} = $hash{foo}";

$hash{foo} += $string;
print "\$hash{foo} = $hash{foo}";

$string = '2e-36';
$hash{foo} += $string;
print "\$hash{foo} = $hash{foo}";

__END__

$hash{foo} = 1e-149
$hash{foo} = 3e-149
$hash{foo} = 2e-36
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