open my $in, '<', 'infile.txt' or die "Bummer: $!";
open my $out, '>', 'outfile.txt' or die "What: $!";

while(my $line = <$in>){
    if(my ($epoch) = $line =~ /^#\+(\d+)\s*$/){
        my $human_readable = `date -d \@$epoch`; # or:
        #     my $human_readable = scalar localtime $epoch;
        print $out $human_readable;
    }
    else{
        print $out $line;
    }
}