If you open up the source code (the code is in Net::SNMP::PDU), you will see that the module is expecting the input to match

/^\.?\d+\.\d+(?:\.\d+)*/
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And the format that you commented out was just fine.

use strict;

my $a = '1.3.6.1.2.1.1.3.0';
if ($a =~ /^\.?\d+\.\d+(?:\.\d+)*/) {
    print "match\n";
}
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