use strict;
use warnings;

open my $fh, "<", "file.frg";

while(<$fh>) {
    if (/^{FRG/../^}/){
        if (/^acc:(\d+)/) {
            print ">$1\n";
        }
        elsif ( /^seq:/ ) {
            while(<$fh>) {
                last if /^\./;
                print;
            }
        }
    }
}