here is the documentation to the module Bio::SeqIO
The subseq function requires a beginning and an end to what it is going to extract so probably you want to reformat your coordinates as follows
$seq->subseq(coordinate-1, coordinate+1)to extract three bases with the middle one being the actual nucleotide you want to compare your alleles against.
Note that BioPerl can be a bit slow and therefore consider using something like sfetch or esl-sfetch which will again require providing beginning and end positions to what it is to extract
David R. Gergen said "We know that second terms have historically been marred by hubris and by scandal." and I am a four y.o. monk...