How To Merge Fasta Header With Description Using Perl :

I have a fasta file that contains sequences like so :

>PT_TAC_1
ATGCATGCATGC>PT_TAC_2
ATGCATCGATCG

Following BLASTx, i obtained blast results in tabular format. The Sequence ID is in one column and the corresponding description is in the other, like so :

ID          Description
PT_TAC_1   Some_unique_description

How do i merge the description so as to result in a fasta file containing :

>PT_TAC_1_some_unique_description
ATGCATGCATGC