Something like this perhaps?

#!/usr/bin/perl

use strict;

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